1. Field of the Invention
The present invention pertains to downhole assemblies having friction reducing elements. More particularly, the present invention pertains to dowhole assemblies having at least one friction reducing element on at least one exterior surface to reduce frictional forces acting on said assemblies. More particularly still, the present invention pertains to downhole assemblies having at least one ball transfer unit on at least one exterior surface.
2. Brief Description of the Prior Art
During operations in the oil and gas industry, a pipe or substantially tubular tool having a certain outer diameter is frequently inserted or otherwise received within a wellbore or pipe having a larger inner diameter. Such operations are often performed during both drilling and production phases of a well's life cycle. By way of illustration, but not limitation, drill pipe, casing and other tubular goods are routinely run into cased and uncased wellbores. Similarly, tools and other downhole assemblies are often conveyed in and out of wells via jointed pipe, continuous (spooled) tubing and/or wireline.
In such cases, frictional forces—typically so-called “sliding friction—from surrounding surfaces acting on such inner pipe or tool(s) can impede such inner pipe or tool(s) from advancing further into a well. In order to reduce or lessen such frictional forces, friction reducing assemblies can be utilized. In most cases, such friction reducing assemblies are installed on at least one outer surface of a tubular or assembly that is being conveyed into a well.
Various types of friction-reducing devices including, without limitation, so-called “roller subs,” are currently used throughout the oil and gas industry in an effort to reduce or limit such sliding friction. Conventional roller subs typically comprise substantially cylindrical body members having radial slots at predetermined intervals. Rollers and/or wheels are rotatably disposed within said slots. In many cases, such roller subs are modular in construction; as such, the roller subs can be assembled in a manner to secure the rollers and/or roller wheels in place, avoiding the need for grub screws and facilitating relatively compact dimensions.
Although such devices can be used in many different applications, use of roller subs is especially prevalent during downhole wireline operations. Such wireline operations generally rely on gravity alone in order to advance a toolstring in a well. As such, roller subs can be especially useful in directional wellbores that deviate substantially from vertical. Roller subs help to reduce sliding friction, thereby allowing wireline tools to advance deeper within a well.
During drilling of a well, stabilizers may be coupled into a drill string to bear against a wall of a drilled hole and thus centralize a drill string in a wellbore. After a well has been drilled, devices known as centralizers may also be employed to centrally locate a liner or casing within a well and provide “stand off” from a surrounding wellbore or casing. Conventional roller subs are also used to reduce sliding friction associated with devices such as stabilizers and centralizers. However, such conventional roller subs are typically not adjustable to various conditions within a well such as, for example, wells having multiple internal diameters.
One type of conventional stabilizer rolling sub assembly comprises rollers mounted on multiple axes, which are each parallel to a longitudinal axis of a stabilizer body (and, hence, also parallel to the longitudinal axis of a drillstring and of a well drilled thereby). The functional effect associated with this form of stabilizer is to reduce rotational friction, while having a neutral or even adverse effect on longitudinal sliding frictional forces.
Conventional roller subs are also sometimes run in connection with bow spring centralizers to reduce sliding friction. However, such bow spring centralizers are typically not sturdy enough to support the full weight of tubing, casing or other pipe strings. Additionally, such conventional centralizer roller subs typically include axles, grub screws and/or other mechanical components that are especially vulnerable to failure during use.
Thus, there is a need for a friction reducing apparatus that significantly reduces frictional forces acting upon tubulars, tool strings, centralizers and/or other downhole assemblies. Such friction reducing apparatus must reduce or diminish frictional forces including, without limitation, sliding friction, generated between outer surface(s) of a pipe or tool, and inner surfaces of a surrounding wellbore or tubular. Among other benefits, the friction reducing apparatus should beneficially assist pipe or downhole tools to traverse obstructions or so-called “tight spots” within a wellbore, minimize or eliminate damage caused when said pipes or downhole tools scrape against surrounding wellbore walls, and reduce rotational friction forces and pipe torsional strain.